1. Field of the Invention
The present invention relates to a subsurface safety valve used for controlling fluid flow in a well conduit and, more particularly, to a pressure equalizing subsurface safety valve having a fluid injection system.
2. Description of the Related Art
Subsurface safety valves are commonly used in wells to prevent uncontrolled fluid flow through the well in the event of an emergency, such as to prevent a well blowout. Conventional safety valves use a flapper which is biased by a spring to a normally closed position, but is retained in an open position by the application of hydraulic fluid from the earth's surface. A typical subsurface safety valve is shown and described in U.S. Pat. No. 4,161,219, which is commonly assigned hereto.
One problem associated with subsurface safety valves relates to the corrosive effect of well fluids on the material from which subsurface safety valves have been made. One solution to this problem is to make the safety valve from materials that are resistant to such corrosion. The drawback to this solution is that, due to the greater expense of noble metals, the cost of the safety valve is much greater than before. An alternative to making the safety valve from these costlier materials is to inject various chemicals into the safety valve. The chemicals mix with the production fluid and protect the safety valve. The injection fluids may be chemicals such as for retarding corrosion, scale, parafin, asphaltenes, and the like. It is therefore an object of the present invention to provide an improved subsurface safety valve having a means for injecting fluids into the safety valve.
Another problem associated with subsurface safety valves may arise when moving the valve closure member from its closed to its open position. When the closure member is in the closed position, well fluid pressure below the closure member acting upon a relatively large surface area of the closure member makes opening of the closure member difficult. This difficulty in opening cannot be easily overcome simply by increasing the force exerted against the closure member by an opening piston and cylinder assembly because the relatively small cross-sectional area of the opening piston and cylinder assembly would require a fluid pressure that may burst the control line carrying hydraulic fluid from the earth's surface to the piston and cylinder assembly. Additionally, when the closure member is opened the initial flow of well fluid is relatively rapid which tends to etch, or erode, the primary sealing surface of the closure member. Any damage to this primary sealing surface is extremely critical because it is this sealing surface which must be intact to prevent uncontrolled flow of well fluids. The present invention solves these difficulties by providing a subsurface safety valve with an equalizing mechanism to allow the pressure above and below the closure member to equalize prior to the complete opening of the closure member.